The invention relates to the general field of magnetic disks with particular reference to fabricating miniaturized write coils having high aspect ratios.
Referring to FIG. 1, we show, in schematic representation, a cross-sectional view of a write head for a magnetic disk system. The magnetic field needed to perform the write operation is generated by flat coil 16 made up of a number of turns, with 13 being an example of one side of a single turn. Surrounding the flat coil is magnetic material comprising upper and lower pole pieces 12 and 11 respectively. These pole pieces are joined at one end (on the left in this figure) and are separated by small gap 14 at the other end. The magnetic field that is generated by flat coil 16 ends up being concentrated at gap 14. It is sufficiently powerful that the fringing field that extends outwards away from gap 14 is capable of magnetizing the magnetic storage medium over whose surface 15 the head xe2x80x98fliesxe2x80x99. The distance between gap 14 and surface 15 is typically between about 0.005 and 0.075 microns.
As write heads continue to develop, a need has arisen for coils that are both more compact as well as having greater current carrying capability. This implies coils that have of the order of 3-20 turns made up of wires whose widths are less than about 0.5 microns and that are separated from one another by less than about 0.5 microns. Thus, to retain adequate cross-sectional area, it is necessary for coils to be at least about 1 microns thick.
Manufacturing coils whose windings have the required high aspect ratios have presented the industry with formidable challenges. Two requirements, inter alia, have to be met. The first is that the material for the coils have little or no built-in stress. The second is that a method other than subtractive (i.e. post deposition) etching be used. Electroplating meets both these requirements. Once a seed layer (almost always of copper) which can be connected to a source of electrical power is available on a substrate, it becomes possible to place a non-conductive mask on the seed layer and to then build up the uncovered areas through electroplating.
Once the coil has been electroformed, it is still necessary to remove all seed layer material that is not directly beneath the coil. This is a particularly difficult problem for copper when such high aspect ratios and narrow separations between windings are involved. Since both the seed layer and the coil are made of copper, a highly anisotropic etching process must be used to ensure that no vertical surfaces (such as the sides of the copper coil) are attacked, etching being limited to horizontal surfaces. This means that reactive ion etching (RIE), or similar process, must be used. RIE, in turn, depends on the generation of gaseous reaction products, which is not the case for copper.
Until recently this problem has precluded routine use of RIE for copper etching. However, a method for overcoming this was recently proposed by Yue Kuo and S. Lee in xe2x80x9cA new copper reactive ion etching processxe2x80x9d in the Proceedings of the ""99 Joint International Meeting of the ECS, ECSJPN, and JPN SAP (Nov. 1, 1999). Their method relies on the formation of copper chloride which can later be selectively removed using a liquid etchant. Although this represents an improvement over earlier practices, problems still remain. In particular, some of the copper coil itself is also converted to copper chloride, thereby reducing the coil thickness, and a certain amount of copper chloride remains behind despite the use of the liquid etchant. The present invention provides solutions to both these problems.
An alternative approach to the blanket seed layer method described above is the damascene method in which the photoresist frame is formed before deposition of the seed layer. For a comparison of the blanket seed and damascene methods we refer now to FIGS. 2a and 2b. Seen in both figures is substrate 21 and photoresist frame 23. However, in FIG. 2a the frame sits over the seed layer 22a while in FIG. 2b the seed layer 22b has been deposited over the frame. FIGS. 3a and 3b show the electroplated layers 31a and 31b respectively. 31a is required to end slightly below the top of frame 23 while 31b must be thick enough to fully fill the frame, causing its thickness to be about twice that of 31a. 
FIG. 4a shows the structure after removal of the photoresist and all seed layer not under coil fragment 31a. In FIG. 4b the structure is seen after the surface has been planarized and polished down (generally using chemical mechanical polishing, or CMP) until the top surface of photoresist 23 is exposed, thereby allowing it to be removed, leaving behind the desired coil.
Although the damascene approach solves the problem of how to remove unplated areas of seed layer, it is actually subject to severe limitations. As the aspect ratio of the windings increases and their separation decreases, there is a growing danger of forming voids in the separation area between coils because of loading effects that cause copper on opposing top surfaces to meet before the separation area is fully filled in. Additionally, it is clear that the damascene approach sets limits on the thickness of the seed layer, particularly if it is preceded by a xe2x80x98gluexe2x80x99 layer to improve adhesion of the copper to the substrate. Again, the problem of the formation of xe2x80x98keyholexe2x80x99 voids when filling spaces of high aspect ratio is well known. It is thus clear that flat coils having high aspect ratio and narrow separation between coils will need to be fabricated using the blanket seed approach.
Finally, we note that attempts to remove the seed layer by ion beam milling after formation of the coil, using the coil as its own mask, have been unsuccessful. This is because of shadowing effectsxe2x80x94the high aspect ratio of the coil makes it very difficult for ions to reach the seed layer without first glancing off the side-walls of the coils which causes the removal of substantial amounts of material therefrom.
A routine search for prior art was performed. Although several references of interest were found none describe the exact process that constitutes the present invention. Thus, for example, in U.S. Pat. No. 5,926,349, Krounbi et al. teach a plate-up coil process. Coil manufacturing processes based on electroplating are disclosed in U.S. Pat. No. 5,777,824 (Gray) and U.S. Pat. No. 5,684,660 (Gray et al.), while other plating processes are taught in U.S. Pat. No. 5,875,080 (Seagle) and U.S. Pat. No. 5,751,522 (Yamada et al.).
It has been an object of the present invention to provide a process for manufacturing a flat coil for use in the write head of a magnetic disk system.
Another object of the invention has been to be able to manufacture flat coils having very high aspect ratios and very small separation between windings.
A further object of the invention has been to manufacture a flat coil that is protected against corrosion failure during its life.
These objects have been achieved by using the blanket seed layer process wherein the coil is electroformed from the seed layer within the confines of a photoresist frame. Key features of the invention include capping the coil with a layer, of gold to protect it during the removal of copper that is exposed after photoresist removal, using RIE to convert the exposed copper to the chloride (which is then easily rinsed away), and then removing all last traces of said chlorides through an ashing process.